Bitrate transcoding is a technique which allows a primary video stream encoded at a bitrate BR1 to be converted into a secondary video stream encoded at a bitrate BR2, lower than BR1, the bitrate reduction being performed in order to meet requirements imposed by the means of transport during broadcasting. A transcoding device as described in the opening paragraph is disclosed in European Patent Application n° EP 0690 392 (PHF 94001) and is depicted in FIG. 1. Said device (100) for transcoding encoded digital signals (S1) which are representative of a sequence of images, comprises a decoding channel (11,12) followed by an encoding channel (13,14,15). A prediction channel is connected in cascade between these two channels, and said prediction channel comprises, in series, between two subtractors (101,102), an inverse discrete cosine transform circuit IDCT (16), a picture memory MEM (17), a circuit for motion-compensation MC (18) in view of displacement vectors (V) which are representative of the motion of each image, and a discrete cosine transform circuit DCT (19).